Resizing main widget after removal the internal widget

nammidd

I can’t figure out, why i can’t resize my main widget right after removing the internal widget? And how to make resizing work? Could please you tell me? I wrote an example:

#include <QApplication>
#include <QHBoxLayout>
#include <QWidget>
#include <QGroupBox>
#include <QTimer>
#include <QObject>

int main(int argc, char *argv[]) {
	QApplication a(argc, argv);

	QWidget* main = new QWidget;

	QGroupBox* widget1 = new QGroupBox("Widget 1");
	QGroupBox* widget2 = new QGroupBox("Widget 2");
	widget1->setFixedSize(400, 250);
	widget2->setFixedSize(400, 250);

	QHBoxLayout* grid_layout = new QHBoxLayout;
	grid_layout->addWidget(widget1);
	grid_layout->addWidget(widget2);

	main->setLayout(grid_layout);
	main->show();

	QTimer::singleShot(3000, [&](){
		delete widget2;
		main->resize(1, 1); // <-------------| Does not works
	});
	QTimer::singleShot(5000, [&](){
		main->resize(1, 1); // <-------------| Works
	});
	return a.exec();
}

Something like this freezes the program

QTimer::singleShot(3000, [&](){
		delete widget2;
		while (main->size() != QSize(1, 1))
			main->resize(QSize(1, 1));
	});

mrjj

Hi
When you remove something from a layout a re-calculation event is put on the event
queue. So its asynchronous as seen if you do

 QTimer::singleShot(3000, [&](){
        delete widget2;
        a.processEvents();  // allow it to process the events from "delete"
        main->resize(1, 1); // <-------------| Does not works
    });

then resize works. However, using processEvents() is not recommended but you can use a single shot for the
resize and it will work too.

JonB

@mrjj , @nammidd
Are you sure you can just delete a widget on a layout without telling the layout about it? I thought you need to go something like:

grid_layout->removeWidget(widget2);
widget2->deleteLater();

?

mrjj

@JonB
Hi
Yes you are right that the correct way
is to use removeWidget or takeAt.
But apparently plain delete also seems to work, i would guess due to the internal
qobject housekeeping notifications.
But very good point!

JonB

@mrjj
Looking at the code for QLayout::removeWidget(QWidget *widget) at https://code.woboq.org/qt5/qtbase/src/widgets/kernel/qlayout.cpp.html#_ZN7QLayout12removeWidgetEP7QWidget it does:

delete takeAt(i);
invalidate();

so it invalidate()s the layout, which update()/redraws it, which might be good here to get main redrawn/resized correctly?

mrjj

@JonB
Good digging :)
Well i did try to call invalidate but in this case, the result was the same.